Non-human primates may enjoy watching someone else trip on a banana peel, according to new research on laughter, which found that apes might appreciate slapstick humor.

The research also helps to explain the origins of laughter and the social aspects of the behavior.

Robin Dunbar, who co-authored one such study with Guillaume Dezecache, described what non-human primates might be amused by.

"The use of language-based jokes is clearly unique to humans," Dunbar, a professor of evolutionary psychology at the University of Oxford, told Discovery News. "There is some suggestion that apes 'play practical jokes' or laugh at another's misfortune, such as the banana skin situation, but these are only casual observations."

"Human laughter derives from the play invitation vocalizations of Old World monkeys and apes, but this is normally confined to juveniles and adolescents; adults don't play," he continued.

"In apes, this is identifiably rather closer to human laughter," Dunbar explained, "and bonobos in particular use laughter a lot in play contexts, even among adults. What seems to have happened is that humans have taken these monkey/ape play vocalizations and tweaked them and increased the frequency of their use."

Human laughter still has an animalistic quality, in the sense that it involves a series of rapid exhalation-inhalation cycles comparable to other primate sounds; it's louder than human speech; and, like sneezing, laughter is contagious.

Although a room full of people can laugh at one comic's joke, Dunbar and Dezecache suspected that the size of bonded natural laughter groups might be limited and similar to social grooming. The latter facilitates bonding and makes individuals feel good, promoting connections between others. Laughter can function in a similar way.

For the study, accepted for publication in the journal Evolution and Human Behavior, the researchers studied multiple social groups in bars throughout the United Kingdom, France and Germany. They took note of conversational subgroup size and laughter subgroup size, meaning the number of individuals laughing in an obviously coordinated way.

The scientists found that laughter groups were limited to three to four individuals.

"We think laughter long predates the appearance of language in human evolution, and was co-opted from play as a mechanism to allow bonding between larger numbers of individuals," Dunbar explained. "Laughter triggers the release of endorphins, which are the neurochemicals used in bonding in monkeys and apes. Laughter allows us to increase the size of the bonding group because several people can laugh together; whereas grooming is, even in humans, a one-to-one activity, with only the recipient gaining the benefit of the endorphins."

In intimate social gatherings, people tend to laugh in sync. This doesn't happen as precisely in larger venues or even when people in different houses are responding to the same jokes on a humorous television program.

Natural laughter group size probably hasn't increased tremendously because these more modern forms of entertainment would not have had time to affect human evolution much.

"What happens in comedy clubs is that we think we are in a small virtual group with the comedian and the people immediately with us," Dunbar noted, "which might explain why not everyone laughs at the same time in a club or theater."

Marina Davila-Ross, a senior lecturer in psychology at the University of Portsmouth, has studied laughter and smiles in apes and humans, and how such communication evolved.

"It is surprising that our group size limit for sharing laughter is generally three, especially when one considers its contagious quality," she told Discovery News.

"Nonetheless, this sharing is likely to help people to bond, notably more so than apes, who share laughter with only one other individual," she added. "Interestingly, laughter has more of a social role in great ape species that live in larger social communities, a pattern that coincides with the findings on human laughter by Dezecache and Dunbar."